Blood pump bearing system

ABSTRACT

An apparatus is are provided for a blood pump bearing system within a pump housing to support long-term high-speed rotation of a rotor with an impeller blade having a plurality of individual magnets disposed thereon to provide a small radial air gap between the magnets and a stator of less than 0.025 inches. The bearing system may be mounted within a flow straightener, diffuser, or other pump element to support the shaft of a pump rotor. The bearing system includes a zirconia shaft having a radiused end. The radiused end has a first radius selected to be about three times greater than the radius of the zirconia shaft. The radiused end of the zirconia shaft engages a flat sapphire endstone. Due to the relative hardness of these materials a flat is quickly produced during break-in on the zirconia radiused end of precisely the size necessary to support thrust loads whereupon wear substantially ceases. Due to the selection of the first radius, the change in shaft end-play during pump break-in is limited to a total desired end-play of less than about 0.010 inches. Radial loads are supported by an olive hole ring jewel that makes near line contact around the circumference of the shaft to support high speed rotation with little friction. The width of olive hole ring jewel is small to allow heat to conduct through to thereby prevent heat build-up in the bearing. A void defined by the bearing elements may fill with blood that then coagulates within the void. The coagulated blood is then conformed to the shape of the bearing surfaces.

ORIGIN OF THE INVENTION

This application is a division of application Ser. No. 08/644,579, filedJun. 17, 1996 now U.S. Pat. No. 5,957,672.

This application is a continuation-in-part of U.S. application Ser. No.08/153,595 filed Nov. 10, 1993 now U.S. Pat. No. 5,527,159, and of U.S.application Ser. No. 08/451,709 filed May 26, 1995 now U.S. Pat. No.5,678,308. The invention described herein was made in the performance ofwork under a NASA contract and is subject to the provisions of Section305 of the National Aeronautics and Space Act of 1958, Public Law 85-568(72 Stat. 435; 42 U.S.C. 2457).

1. Technical Field

The present invention relates generally to rotary blood pumps. Morespecifically, the present invention relates to a ventricle assist devicehaving a bearing system for supporting long-term high speed rotorrotation with minimal friction and heat build-up.

2. Background Art

Ventricle assist devices are frequently used to boost blood circulationto assist a heart which still functions but is not pumping sufficientblood for adequate circulation. The estimated need for a reliablelong-term ventricle assist device (VAD) is presently projected atbetween 50,000 and 100,000 patients per year in the United States alone.

At the present time, rotary blood pumps are often the preferred type ofpump for use as a ventricle assist device as compared to other morecomplex types of pumps which may use pistons, rollers, diaphragms,compliance chambers, and so forth. This is at least partially becauserotary pumps may be manufactured in larger numbers at a relatively lowercost and are typically less complex than other types of pumps. The morecomplex pumps, on the other hand, may cost up to $50,000 per unit.Furthermore, availability of large numbers of complex pumps, as isrequired by the sizeable population that could benefit from such pumps,is limited by high manufacturing, operating, and maintenance costs.Therefore, rotary blood pumps are increasingly used not only forventricular assist applications, but also for cardiopulmonary bypassprocedures and percutaneous cardiopulmonary support applications inemergency cases.

Clinical uses of rotary pumps are normally limited to a few days owingto shortcomings of the devices. A non-comprehensive list of suchproblems or shortcomings would include the following: (1) blood damagewhich may occur when blood comes into contact with rotor bearings due tobearing heat or being forced through small clearances, (2) the need forbearing purge systems which may require percutaneous (through the skin)saline solution pump systems, (3) bearing seizure resulting from theconsiderable thrust and torque loads, or from dried blood sticking onthe bearing surfaces, (4) problems of blood damage (hemolysis) and bloodclotting (thrombosis) caused by relative rotational movement of thecomponents of the pump, (5) pump and control size and shape limitationsnecessary for implantation or convenient mobility, (6) weightlimitations for implantation to avoid tearing of implant grafts due toinertia of sudden movement, (7) difficulty in coordinating andoptimizing the many pump design parameters which may affect hemolysis,(8) high power consumption that requires a larger power supply, (9)motor inefficiency caused by a large air gap between motor windings anddrive magnets, (10) heat flow from the device to the body, (11) complexHall Effect sensors/electronics for rotary control, (12) the substantialdesire for minimizing percutaneous (through the skin) insertionsincluding support lines and tubes, (13) large pump and related hoseinternal volume which may cause an initial shock when filled with salinesolution while starting the pump, and other problems.

Existing bearing systems for externally used rotary blood pumps may havesmall rolling element bearings such as ball bearings. Rolling elementbearings require a shaft seal to prevent blood entering the bearingvoids between the rolling elements. If blood enters the bearing voids,it coagulates and may cause bearing seizure by interfering with therolling elements. Shaft seals complicate pump design, decrease pumpreliability, and reduce pump life.

Some implantable blood pumps utilize pivot bearings. Pivot bearings canoperate immersed in blood without a blood seal. However, to maintain theprecise rotation required in blood pumps to minimize red blood celldamage, such pivot bearings utilize complicated shaft pre-loadmechanisms to eliminate shaft end-play. Shaft pre-load mechanisms areprone to seizure by coagulated blood. They also increase bearing wear.

Other blood pump bearing systems utilize journal bearings flushed withfluids such as saline solution or blood. Journal bearings have minimalwear, but require a separate thrust bearing that complicates pumpdesign. Journal bearings require fluid pressure to support the loads. Ifthe pump utilizes saline solution rather than blood as the bearingfluid, then pump design is significantly complicated by the need for aseparate reservoir, flow lines, and the like. If the pump utilizes bloodas bearing fluid, then potential pump seizure caused by coagulated bloodis a serious concern. In addition, blood flow through a journal bearingis exposed to a high shear environment. The high shear environment maydamage the blood or generate micro-clots that are washed into thepatients blood stream. Finally, journal bearings of the size used inblood pumps require very precise alignment that increase manufacturingcomplexity, and increase costs.

Although a significant amount of effort has been applied to solving theproblems associated with rotary pumps, there is still a great demand fora safe, reliable, and durable blood pump that may be used for longerterm applications.

The following patents describe attempts made to solve problemsassociated with rotary blood pumps including ventricle assist devices.

U.S. Pat. No. 4,625,712 to R. K. Wampler discloses a full-flow cardiacassist device for cardiogenic shock patients which may be inserted intothe heart through the femoral artery and driven via flexible cable froman external power source. A catheter attached to the pump supplies thepump bearings with a blood-compatible purge fluid to prevent thrombusformation and introduction of blood elements between rotating andstationary elements. Due to the very small diameter of the pump,rotational speeds on the order of 10,000 to 20,000 rpm are used toproduce a blood flow on the order of about four liters per minute.

U.S. Pat. No. 4,957,504 to W. M. Chardack discloses an implantable bloodpump for providing either continuous or pulsatile blood flow to theheart. The pump includes a stator having a cylindrical opening, anannular array of electromagnets disposed in a circle about the statorconcentric with the cylindrical opening, a bearing carried by the statorand extending across the cylindrical opening, and a rotor supported bythe bearing. The rotor is in the form of an Archimedes screw and has apermanent magnet in its periphery which lies in the same plane as thecircular array of electromagnets and is driven in stepper motor fashion.

U.S. Pat. No. 4,944,722 to J. W. Carriker discloses a percutaneouslyinsertable intravascular axial flow blood pump with a rotor extensionand drive cable fitting so designed that the thrust bearing surfaces ofthe purge seal and cable fitting can be pre-loaded.

U.S. Pat. No. 4,817,586 to R. K. Wampler discloses an intravascular flowblood pump with reduced diameter having blood exit apertures in thecylindrical outside wall of the pump housing between the rotor bladesand the rotor journal bearing.

U.S. Pat. No. 4,908,012 to Moise et al. discloses an implantableventricular assist system having a high-speed axial flow blood pump. Thepump includes a blood tube in which the pump rotor and stator arecoaxially contained, and a motor stator surrounding the blood duct. Apermanent magnet motor rotor is integral with the pump rotor. Purgefluid for the hydrodynamic bearings of the device and power for themotor are preferably percutaneously introduced from extra-corporealsources worn by the patient.

U.S. Pat. No. 4,779,614 to J. C. Moise discloses an implantable axialflow blood pump which includes a magnetically suspended rotor ofrelative small diameter disposed without bearings in a cylindrical bloodconduit. Neodymium-boron-iron rotor magnets allow a substantial gapbetween the static motor armature and the rotor. Magnetically permeablestrips in opposite ends of the pump stator blades transmit to Hallsensors variations in an annular magnetic field surrounding the rotoradjacent the ends of the pump stator blades.

U.S. Pat. No. 5,049,134 to Golding et al. discloses a seal freecentrifugal impeller supported in a pump housing by fluid bearingsthrough which a blood flow passageway is provided.

U.S. Pat. No. 4,382,199 to M. S. Isacson discloses a hydrodynamicbearing system for use with a left ventricle assist device. The bearingsare formed by the fluid in the gap between the rotor and the stator.

U.S. Pat. No. 4,135,253 to Reich et al. discloses a centrifugal bloodpump provided with a magnetic drive system which permits a synchronousmagnetic coupling with a separate power unit disposed immediatelyadjacent the pump housing but outside of the skin surface. The pump hasa single moving part which includes the combination of an impellerconnected to a magnetic drive rotor. The magnetic drive system floats ona fluid surface of saline solution.

U.S. Pat. No. 4,507,048 to J. Belenger discloses a centrifugal bloodpump with a bell shaped housing having a suction inlet at the apex and atangential outlet adjacent the base. A conical rotator is driven byspaced permanent magnets embedded in the base of the rotator and anexternally generated rotating magnetic field.

U.S. Pat. No. 4,688,998 to Olsen et al. discloses a pump with amagnetically suspended and magnetically rotated impeller. The impellermay be configured for axial flow with a hollow, cylindrical-typeimpeller with impeller vanes on the internal surface thereof. Theimpeller includes a plurality of internally embedded, permanent magnetsthat cooperate with electromagnets for drive and position control of theimpeller.

U.S. Pat. No. 4,763,032 to Bramm et al. discloses a magnetic rotorbearing for suspending a rotor for an axial or radial-centrifugal bloodpump in a contact-free manner, and comprising a permanent andelectromagnetic arrangement.

U.S. Pat. No. 4,846,152 to Wampler et al. discloses a miniaturehigh-speed intravascular blood pump with two rows of rotor blades and asingle row of stator blades within a tubular housing. The rotor's firstrow has no provision for a variable ditch blade but produces a mixedcentrifugal and axial flow by increasing hub diameter. The rotor'ssecond row, axially spaced and having an axial distance between thefirst row, produces a purely axial flow. The stator blades are reversetwisted to straighten and slow the blood flow.

U.S. Pat. No. 4,944,748 to Branim et al. discloses an impeller in ablood pump supported by permanent magnets on the impeller and pumphousing and stabilized by an electromagnet on the housing. The impelleris rotated magnetically and stator coils in the housing are suppliedwith electric currents having a frequency and amplitude adjusted inrelation to blood pressure at the pump inlet.

U.S. Pat. No. 4,994,078 to R. K. Jarvik discloses an electricallypowered rotary hydrodynamic pump having motor windings and laminationsdisposed radially about an annular blood channel and having a motorrotor disposed therein such that an annular blood channel passes throughthe gap between the motor rotor and the windings.

U.S. Pat. No. 5,055,005 to Kletschka discloses a fluid pump with anelectromagnetically driven rotary impeller levitated by localizedopposed fluid forces.

In spite of the effort evidenced by the above patents, there remains theneed for an improved rotary pump for use as a ventricle assist devicethat is reliable, compact, requires limited percutaneous insertions, andproduces fewer blood damage problems such as hemolysis and thrombosis. Abearing system for an improved rotary pump should reliably and preciselysupport the rotor for long-term, maintenance free, low frictionoperation without the need for bearing seals and lubrication. Thoseskilled in the art will appreciate the present invention which addressesthese and other problems.

STATEMENT OF THE INVENTION

The present invention provides a blood pump bearing system for preciselysupporting high-speed rotation of a rotor within a housing through whichblood is pumped. The bearing system includes a shaft to support therotor. The shaft has a circumference defined by a first radius. Theshaft terminates at a shaft end that has a radiused surface defined by asecond radius. The second radius is greater than the first radius andpreferably is about three times greater. A shaft end bearing surfacemounted to the housing operates to contact the shaft end for supportingaxial loads on the shaft. The shaft end bearing surface and the shaftend form a pivot bearing. A shaft circumferential bearing mounted to thehousing has an aperture therethrough for receiving the circumference ofthe shaft. The shaft circumferential bearing is operable to supportradial loads on the shaft. The shaft circumferential bearing preventsradial but not rotational movement between the shaft end bearing surfaceand the shaft end that comprise the pivot bearing.

The shaft end bearing surface is preferably comprised of a firstmaterial that is harder than a second material that forms the shaft end.For this reason, wear occurs on the shaft end to quickly produce a flatsurface on the radiused surface of the shaft end. After the flat surfaceis large enough to support axial thrust loads, wear substantially ceasesto occur. At that time, the flat surface preferably has a diameter ofless than about 0.025 inches. The second radius is selected such thatthe change of shaft axial end-play due to creation of the flat surfaceis preferably less than about 0.005 inches. The total shaft axialend-play after break-in is preferably less than about 0.010 inches.

The shaft end bearing surface includes a flat planar surface thatcontacts the radiused surface of the shaft end prior to break-in of thebearing and that contacts the created flat surface after bearingbreak-in.

The shaft circumferential bearing further comprises a curved innersurface that defines the aperture for receiving the shaft. This curvedinner surface makes line contact along the circumference of the shaft.This line contact is very small to thereby minimize friction and heatbuild-up in the blood pump bearing system as the rotor rotates. Thelocus of line contact has a width of less than about 0.010 inches. Theshaft circumferential bearing may comprise a commercially availablejewel typically known as an olive hole ring jewel by those skilled inthe art. The jewel bearing preferably has a substantiallydoughnut-shaped or toroidal curved inner surface portion that definesthe aperture. The olive hole ring jewel preferably has a width asmeasured parallel to an axis of the shaft that is as narrow as possibleto allow for heat transfer through the bearing. Preferably the width isless than about 0.035 inches and tailor bearing jewels may be obtainedwith significantly smaller widths. The radial clearance between theshaft circumference bearing and the circumference of the shaft is about0.0001-0.0002 inches.

A void is formed between the shaft end and the shaft end bearing intowhich blood may leak between the circumference of the shaft and theshaft circumference bearing. This blood coagulates and becomes trappedin the void. Additional blood is then prevented from flowing into thevoid.

The blood pump bearing system shaft may be integral to the rotorsection, it may be a single separate shaft mounted at both ends, or itmay comprise two end sections.

The present invention also provides a method and apparatus for a rotaryblood pump electromagnetic drive and includes a pump housing defining ablood flow path therethrough. A first stator having a first stator fieldwinding is used to produce a first stator magnetic field. A first rotoris mounted within the pump housing for rotation in response to the firststator magnetic field. The first rotor carries a blade thereon to propelblood through the pump housing along the blood flow path. At least onemagnet is secured to the first rotor and produces a first rotor magneticfield that passes through the first stator field winding during rotationof the first rotor to thereby induce a back emf within the first statorfield winding. Back emf sensor circuitry connecting to the first statorfield winding senses back emf produced during the rotation of the firstrotor within the pump housing. In one embodiment of the presentinvention, at least one magnet is implanted in each of a plurality ofimpeller blades to produce rotational movement of the rotor.

The rotary pump also includes, in a preferred embodiment, an inducerportion of the rotor having a plurality of inducer blades equidistantlydisposed about a circumference of the rotor with each inducer bladehaving a variable pitch along its axial length. An interconnection bladeportion connects at least one of the inducer blades to at least oneimpeller blade to form a continuous blade extending through the inducerand impeller portions of the rotor.

An object of the present invention is to provide an improved rotaryblood pump.

Another object of the present invention is to provide axial and radialsupport of a high-speed spinning impeller used to increase or sustainblood flow for a cardiac patient.

A further object is to provide an improved control circuit forcontrolling a rotor within the pump in response to a back emf producedin stator windings.

Another object is to provide an improved rotor bearing for whichcross-linked blood forms a bearing surface and blood seal.

Yet another object of the present invention is to provide a method foroptimizing pump parameters to reduce blood hemolysis to minimum.

Yet another object of the present invention is to provide a highlyreliable, low friction bearing to support the rotor for high speedoperation while limiting bearing temperature without the need forbearing lubrication.

A feature of the present invention is a reduced air gap betweenpermanent magnets on the rotor and the stator winding.

Another feature of the present invention is an impeller having avariable pitch blade.

Another feature of the present invention is a back emf integratedcircuit for controlling rotor operation.

Yet another feature of the present invention is a rotor shaft having aradiused end with a radius greater than the radius of the shaftcircumference and preferably about three times greater.

Yet another feature of the present invention is an olive hole ring jewelto support the rotor shaft along a low friction, low heat, near linecontact surface around the circumference of the rotor shaft.

Yet another feature of the present invention is a near point contactbetween the radiused end of the rotor shaft and a flat endstone producedwhen the radiused end is worn during break-in of the pump to form asmall flat of the precise size necessary to support thrust loads afterwhich wear substantially ceases thereafter.

An advantage of the present invention is improved rotor control.

Another advantage of the present invention is quantifiably reduceddamage to blood.

Yet another advantage of the present invention is an elimination of theneed for a bearing purge system requiring saline carrying tubespenetrating through the skin.

Yet another advantage of the present invention is the avoidance of theneed for bearing seals, blood forced through close tolerance movingcomponents, excessive bearing heat, corrosive bearing components,pre-load mechanisms, and bearing lubrication.

Other objects, features and intended advantages of the present inventionwill be readily apparent by the references to the following detaileddescription in connection with the accompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view, partially in section, of a rotary bloodpump in accord with the present invention;

FIG. 2 is an elevational view of a stator showing stator laminationsstacked together to form a skewed stator;

FIG. 2A is an elevational view indicating the skewed path of statorfield windings through the stator;

FIG. 3 is an elevational view, partially in section, of an alternativeembodiment rotary blood pump having distinct impeller and inducerblades;

FIG. 4 is an elevational view, in section, of a portion of an impellershowing non-radiused blade tips;

FIG. 5 is a block diagram of a control system including a back emfintegrated circuit and a microprocessor;

FIG. 6A is an elevational view, partially in section, of a ball-socketrotor bearing having a bearing chamber filled with bio-compatiblematerial;

FIG. 6B is an elevational view, partially in section, of a shaft journalrotor bearing having a bearing chamber filled with bio-compatiblematerial;

FIG. 6C is an elevational view, partially in section, having a rotorbearing washed from increased blood flow caused by bend in the pumphousing;

FIG. 6D is an elevational view, partially in section, showing across-section of a rotor bearing shaft having blood flow passages alongthe shaft periphery;

FIG. 6E is a cross-sectional view along line 6E--6E;

FIG. 6F is an elevational view, partially in section, of a rotor bearingfor supporting the rotor in cantilevered fashion;

FIG. 7 is a chart showing optimum pump parameter components determinedfrom methods of optimizing pump parameters to minimize hemolysis;

FIG. 8 shows the test matrix of the present invention for optimizingpump parameters to minimize hemolysis while maximizing pump efficiency;

FIG. 9 is a graph showing change in inducer blade pitch along the axiallength of the inducer;

FIG. 10 is an elevational view, in section, of two axially spaced pumpsfor separate or combined operation in accord with the present invention;

FIG. 11 is an elevational view, partially in section, of components of abearing system in accord with the present invention;

FIG. 12 is an elevational view, partially in section, of components ofthe invention supporting a blood pump rotor in accord with the presentinvention;

FIG. 12A is a cross-sectional view along the line 12A--12A;

FIG. 13 is an enlarged elevational view, partially in section, of oneend of a rotor bearing system in accord with the present invention;

FIG. 14A is an enlarged elevational view, in section, showing pointcontact at end of the radiused shaft prior to pump break-in; and

FIG. 14B is an enlarged elevational view, in section, showing a flatworn on the radiused shaft after pump break-in in accord with thepresent invention.

While the invention will be described in connection with the presentlypreferred embodiments, it will be understood that it is not intended tolimit the invention to these embodiments. On the contrary, it isintended to cover all alternatives, modifications, and equivalents asmay be included in the spirit of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention describes a rotary blood pump which has animproved rotor control system. The rotary pump has pump blade geometryoptimized by a method of the present invention to provide high pumpefficiency while minimizing hemolysis and thrombus (hemolysis is definedquantitatively hereinafter). The pump requires less than 10 watts ofpower to pump 5 liters/minute against a pressure head of 100 mm Hg. Apreferred embodiment of the pump weighs 53 grams and has a length of 75mm and a diameter of 25 mm. An index of hemolysis of from 0.003 to 0.005g/100 liters pumped has been achieved, although using the method of thisinvention, further reductions are possible. For reference, a standardroller pump has an index of hemolysis of 0.06 g/100 liters. Publishedarticles concerning aspects of the present invention are herebyincorporated by reference and include the following: (1) "In VitroPerformance of the Baylor/NASA Axial Flow Pump", Artificial Organs, 1993Volume 17, number 7, page 609-613; (2) "Development of Baylor/NASA axialflow VAD, Artificial Organs, 1993, Volume 17, page 469.

Referring now to the drawings, and more particularly to FIG. 1, there isa shown a rotary blood pump 10 in accord with the present invention.Blood pump 10 includes a preferably metallic tubular pump housing 12which is, in a preferred embodiment, a straight-sided cylinder. Pumphousing 12 has a smooth inner bore wall 15 to minimize thrombusformation. Pump housing 12 defines an axial blood flow path 13therethrough in the direction indicated by blood flow arrows shown inFIG. 1.

Front and rear clamps 14 and 16, respectively, are used to secure flowstraightener 18 and diffuser 20 within pump housing 12. Pump housing 12is sufficiently thin-walled so that the tightening of clamps 14 and 16with clamp screws 22 locally deforms pump housing 12 about flowstraightener 18 and diffuser 20 to affix these components in position.The clamps provide a very convenient means of securing the rotorassembly. Alternatively, other means for securing these components couldbe used such as spot welding, fasteners, interference fit, and so forth.

Flow straightener 18 serves two basic functions (1) it straightens bloodflow to reduce hemolysis while improving pump efficiency (2) it providesa support structure for front ball-socket bearing assembly 24 asdiscussed hereinafter. By straightening the flow of blood as itinitially flows into the entrance 36 of pump 10, hydraulic efficiency isincreased. Straightening the blood flow reduces turbulence to increasethe pump pressure. FIG. 7 lists optimal values and permissible pumpparameter ranges of values for flow straightener 18 and other pumpcomponents.

Flow straightener 18 preferably has four fixed blades 26 but may havefrom two to four blades. Too many blades impede blood flow while too fewblades reduce pump efficiency. For purposes of lowering thrombosis, thefront edge 28 of blades 26 is sloped from inner housing wall 15 to flowstraightener hub 32 so that blood trauma by contact with blades 26 isminimized. Also to reduce blood trauma, flow straightener hub 32 iscylindrical with a round leading surface 34. Surface 34 may also behyperbolical or generally bullet-shaped for this purpose. An alternativeembodiment flow straightener 18a is shown in FIG. 3 and does not havethe sloping front edge blades.

The preferred angle of attack of blades 26 is 90° i.e. the blades wouldintersect a plane transverse to cylindrical housing 12 at an angle of90°. This reference for the angle of attack or pitch of the blades willbe used throughout this specification.

Flow straightener 18 is preferably metallic but may also be formed ofplastic. If formed of plastic and secured in place by clamp 14, it isnecessary to reinforce flow straightener 18 with, for instance, metallicsupports to prevent plastic creep deformation. The plastic creepdeformation phenomena might otherwise eventually cause flow straightener18 to come loose from clamp 14. Reinforcement is also necessary withrespect to other clamped plastic pump components.

Diffuser 20 also has two basic purposes: (1) it de-accelerates andredirects the outflow at blood flow path exit 40 axially to boost pumpperformance and (2) it serves as a support structure for rear rotorbearing 42. Diffuser 20 preferably has from 5 to 8 fixed blades 38 with6 blades being the presently preferred optimum number. Blades 38 arefixably engaged with pump housing 12 after rear clamp 16 is tightened asby screw 22.

To perform the function of de-acceleration and axial redirection ofblood flow, each diffuser blade 38 has an entrance angle of from about10° to 25° for slowing the blood down and an outlet angle of from about80° to 90° for redirecting blood flow in an axial direction. Presentlypreferred blade geometry is listed in FIG. 7 and includes an entranceangle of 15° and an outlet angle of 90°. Tail cone 44 of diffuser 20 ishyperbolical or generally bullet shaped to reduce turbulence or wake ofblood flow from pump 10 so as to minimize blood damage from suchturbulence. Somewhat surprisingly, it was found that increasing thenumber of fixed blades tends to decrease hemolysis.

Rotor 46 is supported for rotary movement with pump housing 12 by frontand rear bearings 24 and 42, respectively. Rotor 46 is divided into twoportions along its axis based on the type and function of the bladesdisposed thereon. Inducer portion 48 of rotor 46 is disposed in thefront part, with respect to blood flow direction, of rotor 46 closest topump inlet 36. Impeller portion 50 is disposed in the rear part of rotor46 closest to pump outlet 40. It has been found that including aninducer portion in an axial flow pump, along with an impeller portion,significantly reduces hemolysis.

FIG. 3 shows an alternative embodiment pump 10a of the present inventionwhich provides for two distinct sets of axially spaced blades which moreclearly distinguish the inducer portion of the rotor from the impellerportion of the rotor. Corresponding components of pump 10 and 10a aregiven the same number, with the difference of an "a" suffix todistinguish the components for comparison purposes as necessary. Areference to one number is therefore a reference to its correspondingnumber in this specification, unless otherwise stated. Where componentsare substantially different between the two pump versions, completelynew numbers are assigned. In pump 10a, inducer portion 48a is separatedfrom impeller portion 50a of rotor 46a by gap 49a which is preferablyless than 0.10 inches. Inducer blade 52a may be tapered (not shown) atforward end 56a so that blade 52a has a smaller radial length at forwardend 56a, perhaps even blending into hub 73. However, using the method ofthe present invention, it has been found that a continuous blade pumphas even more reduced levels of hemolysis than the non-continuous bladepump 10a. Thus, pump 10 shown in FIG. 1 is the presently preferredembodiment.

Inducer blades 52 on inducer 53 have a variable pitch along their axiallength. FIG. 9 shows an inducer blade angle profile that plots angle ofattack in degrees versus axial position on inducer 53 in inches. It wasfound that the inducer portion 48 reduced hemolysis by approximately 45%from a pump design without the inducer. Hydraulic efficiency was alsoincreased as the rotation speed required to pump 5 liters/min of bloodat 100 mm Hg. dropped from 12,600 rpm to 10,800. The inducer bladespre-rotate the blood before it enters the main pumping blades (i.e.impeller blades 54) to reduce hemolysis.

Inducer blades 52 also achieve a pumping action that effectivelyproduces a two-stage, increased efficiency pump. Optimum inducer bladegeometry for minimal hemolysis and maximum pump efficiency is listed forspecific parts of inducer blade 52 in the chart of FIG. 7. Thus, theentrance angle of leading end 56 of inducer blade 52 is preferably 10°but has a preferred range from about 10° to 20° . The shallow entranceangle effectively engages the blood for movement but does not damage it.The pitch of inducer blade 52 continues to change along its axial lengthand preferably is about 30° at midpoint 58 of the blade. The tailing end60 of inducer blade 52 preferably has an outlet angle of 20°. Thisvariable pitch is described in FIG. 9 which shows how pitch variesversus axial length of inducer 53. As well, it is desired that inducerblades have a wrap of preferably 240° around rotor 46. The presentlypreferred overlap of each blade over other blades is 120° or a 50%overlap. The chart of FIG. 7 provides a complete listing of relevantpump parameter values including preferred ranges of operation. Outsideof these ranges, pump efficiency drops and/or blood damage is morelikely to occur.

Pump 10 includes an interconnecting blade portion 62 which is notincluded in pump 10a. Although the two-stage pump 10a producessignificantly reduced hemolysis and efficient pump operation as comparedto a single stage pump, it has been discovered that by interconnectinginducer blades 52 with impeller blades 54 with interconnecting bladeportion 62, hemolysis may be reduced to even lower levels whilemaintaining efficient pump operation.

Impeller blades 54 on impeller 55 have an entrance angle in leading endregion 64 of preferably 20°. This may be seen more clearly in FIG. 3which has no interconnecting blade portion 62. The entrance anglepreferably smoothly tapers to an optimum preferred outlet angle of 90°at blade tailing end region 66. The optimum ranges of operation for theentrance and outlet angles is given in FIG. 7.

Impeller blades 54 include axially longer impeller blades such as longerblade 68 and axially shorter impeller blades such as shorter blade 70.The alternate long and short blade arrangement on impeller 55accommodates multiple magnetic poles for electric motor operation asdiscussed hereinafter, and still maintains adequate flow area throughimpeller 55. Presently, the preferred number of impeller blades is six,but a range from two to six blades provides a range of permissible pumpefficiency. If impeller 55 included six axially longer blades, such aslonger blade 68, flow area through impeller portion 50 would berestricted to such an extent that the blades actually begin to block theflow they are intended to produce.

Using the method of this invention, it was unexpectedly discovered thathemolysis does not necessarily increase with the number of blades, asanticipated. The alternating long-short blade arrangement of the sixbladed impeller of the present invention does not cause hemolysis anymore significantly than a two-bladed impeller. In some cases, animpeller with four long blades may cause more hemolysis than either atwo or six bladed impeller. It is possible that the degree of hemolysisdepends more on the number of long blades rather than the total numberof blades.

FIG. 4 shows a portion of impeller 55 in cross-section to illustratesubstantially flat, non-radiused blade tips 72. It has been unexpectedlyfound, using the method of this invention, that flat or substantiallyflat, non-radiused blade tips have substantially the same pump responsebut do not produce significantly different results from rounded orradiused blade tips with respect to hemolysis. It was anticipated thatflat blade tips would produce higher hemolysis. Because flat blade tipsare less expensive to manufacture in conjunction with magnets to be usedin the blades as discussed hereinafter, flat blade tips comprise thepresently preferred embodiment. Further test results are discussed ingreater detail in the previously noted articles incorporated herein byreference.

FIG. 4 also illustrates a preferred rotor hub 73 with outside diameter74 compared to overall outside diameter 76 of inducer 53 and/or impeller55. The preferred ratio is 0.48 although a range of 0.45 to 0.55 permitsexcellent pump operation. If the hub is smaller than permitted by thisrange, blood becomes excessively swirled and may tend to recirculatewithin pump 10 in the wrong flow direction to possibly damage the bloodas well as reduce pump efficiency. If the hub is too large so as to beoutside of this range, the hub tends to block flow through pump 10.

The radial clearance 78 between inducer 53 and/or impeller 55 withrespect to pump housing 12 inner wall 15 is preferably in the range ofabout 0.003 inches to 0.015 inches. Using a test matrix as per themethod of this invention, it was unexpectedly discovered that smallerradial clearances lowered hemolysis. It was expected that smallerclearances would produce greater blood damage due to higher sheerstresses on the blood. The presently preferred radial clearance 78 is0.005 inches. Axial clearances between components such as flowstraightener 18, rotor 46, and diffuser 20 are shown in FIG. 7. Axialclearances should be within the ranges shown to improve pump efficiencyand to reduce hemolysis.

In order to reduce air gap between stator 80 and magnets 82, the magnetsare preferably sealingly mounted within impeller blades 54. Reducing theair gap between stator 80 and magnets 82 increases motor efficiency,because magnetic flux is not as diffused as occurs in motor designs withlarge air gaps. The preferred radial spacing or air gap between magnets82 and stator 80 is from 0.01 inches to 0.025 inches. Magnets 82 arepreferably rare earth magnets because of the higher magnetic fluxproduced by such magnets. Each magnet 82 is encapsulated into anindividual pocket 84 to eliminate corrosion. Because magnets areindividualized, motor torque and rotor weight can be easily adjusted inthe manufacturing stage to provide motors that are tailored to the typeof pump performance necessary without producing excessive pump weight.

Field winding 88 generates a magnetic field to rotate rotor 46. Stator80 is comprised of individual stator laminations 86 to eliminate eddycurrents that generate heat and reduce efficiency. Heat flow from pump10 is directed both into the blood stream and into the tissuessurrounding pump 10. In this way, the blood is not heated in a mannerthat may damage the blood and, as well, the surrounding tissues do nothave to absorb all the heat. Heat conductive flow paths using thermallyconductive material, such as the metal of the stator or thermallyconductive gel, may be used to provide approximately the desired ratioof heat flow to tissues compared to the heat flow to the blood.

As illustrated in FIG. 2, stator 80, comprised of individual laminations86, is stacked in a presently preferred skewed manner such that pathways90 provide a motor winding pathway that is offset from the rotor axis92. The skew of laminations 86 may or may not correspond in some mannerwith the offset angle or changing offset angle of the row of magnets 82and is not limited to the position shown in FIG. 2. A skewed stator 80is also indicated in FIG. 2A which shows an offset path from axis 92which field windings 88 travel through stator 80. The skewing angle oroffset from the rotor axis is used to optimize performance. The skewingangle of stator 80 may be variable rather than fixed along its length.While skewed stator 80 is the presently preferred embodiment, otherfactors or combinations of factors (e.g. small air gap, magnetorientation, etc.), as discussed, produce excellent pump and motorperformance without a skewed stator.

An axial force is produced on rotor 46 during rotation which can bevaried by moving stator 80 axially along pump housing 12. Stator 80 isaxially adjustable for this purpose and could be fixed in positionduring manufacturing for optimal performance given the number of magnetsto be used and other factors discussed hereinafter. The axial forceproduced hereby can be used to offset thrust created during pumping toreduce the load on front or rear bearing assemblies 24 and 42,respectively, as desired. The axial positioning of stator 80 may also beused to optimize electrical motor efficiency.

Referring to FIG. 5, a block diagram of control system 100 of thepresent invention is shown. Note that control system 100 may operate twomotors 1 and 2 such as shown in FIG. 10. For some applications eitherfor implantation or for external use, it may be desirable to have twopumps connected either in parallel or series. Thus, control system 100can be easily configured for this purpose if desired. In addition,magnets (not shown) may be placed in the inducer hub to provide asecondary motor in the case of primary motor or controller failure.Various other back-up and redundancy configurations may be used.

For instance, in an axially spaced pump configuration shown in FIG. 10,motors 1 and 2 are axially displaced from each other and may be operatedseparately or in conjunction with each other as control system 100regulates power, as discussed hereinafter, to axially spaced stators 207and 208 containing stator windings 210 and 212, respectively.Ball-socket bearing 203, or another bearing discussed hereinafter, onmodified diffuser 204 rotatably supports rotor 205 of motor 2 which isaxially spaced from rotor 206 of motor 1. Diffuser 204 acts as a flowstraightener when independent operation of motor 2 is desired. Controlsystem 100 may be used to operate both motors simultaneously or to turnone motor on if micro-controller 102 senses a motor has failed.Micro-controller 102 may be programmed for pulsatile motor operation orcontinuous speed motor operation of one or more motors, as desired.

If only one pump is to be used, extra components may be removed. In FIG.5, except for micro-controller 102, most components are duplicated toallow operation of two motors. For convenience, reference tocorresponding components will be made to one number with thecorresponding component having an "a" suffix. Control system 100operates either manually or by micro-control as discussed subsequentlyso it may be used for test purposes if desired.

Control system 100 applies current to stator windings 88. Preferablystator 80 includes three stator windings 88. Stator 80 generates arotating magnetic field which rotor 46, containing magnets 82, followsto produce motion. The motor stator may be three phase "Y" or "Delta"wound. The operation of the brushless D.C. motor of the presentinvention requires a proper sequence of power to be applied to statorwindings 88. Two stator windings 88 have power applied to them at anyone time. By sequencing the voltage on and off to the respective statorwindings 88, a rotating magnetic field is produced.

A preferred embodiment three-phase motor requires a repetitive sequenceof six states to generate a rotating magnetic field but othercommutation schemes could be used as well. The six states are producedby electronic commutation provided by power F.E.T.'s 104. If Motor 1were sequenced through the six electrical states at a controlledfrequency without feedback, its operation would be that of a steppermotor. In such a mode of operation, the motor rapidly loses its abilityto generate torque as the rpm's increase.

Control system 100 detects back electromotive force or back emf tocontrol motor operation. Whenever a conductor, such as field winding 88,is cut by moving magnetic lines of force, such as magnets 82, a voltageis induced. The voltage will increase with rotor speed. It is possibleto sense this voltage in one of the three stator windings 88 becauseonly two of the motor's windings are activated at any one time,determine rotor 46 position, and to activate commutator switches 104.The circuitry is much simpler and more reliable than Hall Effect sensorswhich have been used in the past. Although a back emf control is thepresently preferred embodiment, a Hall effect driven commutation schemecould also be used.

Back emf integrated circuit 106 provides sensors for detecting the backemf from lines indicated at 107 and operates commutation switches 104accordingly. A presently preferred back emf integrated circuit includesa ML4411 motor controller integrated circuit. Each commutation switchF.E.T. is preferably turned all the way on or off for maximum powerefficiency.

Back emf integrated circuit 106 also provides start up mode operationwhen the back emf is not present or large enough for detection. Fromzero to approximately 200 rpm's, motor 1 operates in stepper motorfashion as described hereinbefore. Motor speed is controlled withdifference amplifier 108 which may take its speed signal from eithermicro-controller 102 or speed adjust pot 110 as selected by switch 112.A speed detection signal is available from back emf integrated circuit106 for this purpose.

Restart circuit 110 and micro-controller 102 monitor voltage developedacross sense resistor 111 (present preference is about 0.1 ohms) and thefrequency signal from back emf integrated circuit 106 to determinewhether motor 1 should be restarted i.e. due to sudden increase ordecrease in current or frequency. Switch 113 may be used to selectbetween use of restart circuit 110, micro-controller 102, or a manualrestart switch 114. Controller 102 may be programmed to produce an alarmsignal if there are sudden changes in power consumption or frequency asmay occur if heart strength weakens or improves. To protect theelectronics from electromagnetic interference (EMI), ferrite beads 116are used with wires to an external power supply. The electronics arepreferably hermetically sealed in case 118 which is formed of a high mumaterial to limit EMI.

Referring now to FIG. 11 through FIG. 14B, there is shown a presentlypreferred bearing system embodiment 300 in accord with the presentinvention. While bearing system 300 is exemplified within an axial flowpump, it could also be used in a radial flow pump configuration, such asa centrifuge, or a mixed flow pump. The long life of the bearing systemlends itself to use either in long-term implanted blood pumps orshort-term external devices. While bearing system 300 is especiallysuited for blood pumps, it may also be used in fluid pumps or motors toreliably support high rotor speeds with little friction for long life.Preferably, bearing system 300 is comprised of ceramics, as discussedhereinafter. However, if wanted for particular conditions, bearingsystem 300 may comprise materials such as hardened metals.

Bearing system 300 is comprised of three basic components that include ashaft, an endstone, and an olive ring jewel. Representative elements inFIG. 11 disclose shaft 302, endstone 305, and olive hole ring jewel 307.In the presently preferred embodiment, bearing system 300 utilizes thesame bearing components on front bearing assembly 309 as on rear bearingassembly 311 to thereby support both ends of a rotor, such as rotor 304.However, differences between front bearing assembly and rear bearingassembly are preferably designed to develop after break-in of bearingsystem 300, as discussed hereinafter. Front bearing assembly 309 andrear bearing assembly 311 may be mounted in the pump casing or to pumpflow elements such as the diffuser, straightener, and the like discussedherein. Such mountings are referred to generally in FIG. 12 as component301. Bearing components, such as endstone 305, could be monolithicallyformed within pump flow elements 301 if those elements are formed of thenecessary materials.

Shaft 302 of bearing system 300 may comprise a solid shaft mounted oraffixed through the rotor 304. Alternatively, shaft 302 may be comprisedof two end sections that are mounted within appropriately sized butoppositely situated cavities in rotor 304. In FIG. 13, one of such shaftend segments is denoted as shaft end section 306. As well, shaft 302 maybe monolithically constructed with rotor 304 or a portion thereof sothat it forms a one piece unit.

Preferably, shaft 302 is comprised of zirconia, whereas endstone 305 andolive hole ring jewel 307 are preferably formed of sapphire. The factthat sapphire is a somewhat harder material than zirconia is used toadvantage as discussed subsequently.

In the presently preferred embodiment of shaft 302, radiused endsurfaces 308 and 310 are formed on front and rear bearing assemblies 309and 311, respectively. The remainder of shaft 302 is preferablycylindrical along its length but could have tapering portions orportions with other cross-sectional shapes. Because the sapphirematerial forming endstone 305 is somewhat harder than the zirconia ofshaft 302, radiused surface 308 in front bearing 309 wears but endstone305 experiences substantially no wear. Designing the bearing system towear only on radiused surface 308 is preferable because this designresults in less friction after break-in as compared to designing wear tooccur on endstone 305, or on both endstone 305 and radiused surface 308.Furthermore, because front bearing assembly 309, rather than rearbearing assembly 311, supports the thrust load, rear bearing assembly311 experiences little wear.

The radius of curvature R1 indicated in FIG. 11 of radiused end surface,such as surface 308 as compared to the radius of curvature R2 of thecircumference 312 as shown in FIG. 12 is preferably about three to one.However, any ratio larger than about one to one could be used. Theselection of the radius of curvature R1 in this manner provides that thewear occurs very quickly during initial break-in. In fact, referring toFIG. 14, rapid wear begins almost immediately during initial rotation atcontact point 318 on radiused surface 308 of shaft end section 302 offront bearing assembly 309. Wear then tapers off essentially to zerowear once bearing surface area 320 becomes large enough to support thethrust load. In other words, true point contact occurs during initialbearing operation of front bearing 309, but wear of radiused surface 308results in a small flat or bearing surface area 320 after an initialbreak-in period to effect a near point contact.

While the surface area of point contact 318 between radiused surface 308and endstone 305 may have an area with a diameter of a few angstroms,the diameter D1 of the wear flat or bearing surface 320 is preferablyless than about twenty-five thousandths (0.025) of an inch. Sincelittle, if any, axial load is placed on rear bearing assembly 311,little, if any, wear occurs on the radiused shaft end 310. This is onedistinction between front and rear bearing assemblies 309 and 311,respectively.

The selection of the radius of curvature R1 also affects the change inend-play E1 of shaft 302, shown proportionately overly large in FIG. 12.The change in end-play E1 between before and after break-in is indicatedat least somewhat more proportionately in FIG. 14A and FIG. 14B, wherethe width of the end portion that is worn off to create wear flat orbearing surface 320 is about the same as the line width designated asE1. Preferably, change E1 is designed to be less than five thousandths(0.005) of an inch. As suggested by FIG. 14A that presents a greatlyenlarged view of shaft 302, change E1 is a relatively small change inaxial play. It will be understood that increasing the radius ofcurvature R1 will produce the necessary size bearing surface area 320 tosupport axial thrust with a relatively smaller change E1 in end-play ofshaft 302. Thus, the overall change in length of shaft 302 due to wearis minimized by using a shaft end radius R1 significantly larger thanshaft radius of circumference R2. This results in extremely stable shaftend-play and axial clearances between pump components throughout thelife of the pump. Stable shaft end-play maintains low levels of bloodclotting and damage. Preferably, the shaft end-play after break-in isless than ten thousandths (0.010) of an inch.

The purpose of rear endstone 322 is mainly to secure and minimizeimpeller movement during handling and due to patient mobility. Rearendstone 322 therefore completes bearing system 300 to provide a sturdy,precise, and simple assembly.

Olive hole ring jewels, such as jewel 307, may be commercially purchasedor specially made. Olive hole ring jewels are normally used incomponents of precision instruments such as relatively slow movingvisually readable needle indicators of analog meter movements such asvolt meters, and the like. However, in a preferred embodiment of thepresent invention, olive hole ring jewel 307 is used to support highspeed rotation of shaft 302 with very little friction. Olive hole ringjewel 307 has a doughnut-shaped or torroidial inner surface portion 330that makes line contact, or near line contact, around circumference 312of shaft 302 as indicated in cross-section in FIG. 12A. In FIG. 14A,cross-sectional points 319 and 321 of inner surface portion 330 contactshaft 302. Of course, points 319 and 321 lie on the circle indicated ascircumference 312 of the cross-section shown in FIG. 12A.

Inner surface 330 also defines aperture 334 (see FIG. 11) that receivescircumference 312 of shaft 302. Little bearing heat is produced becauseof the very low friction between zirconia shaft 302 and jewel 307 due tothe near line contact that is made around the circumference 312 of shaft302. The locus of the width of the near line contact, as measuredparallel to the axis of rotor 304, is preferably less than about tenthousandths (0.010) of an inch.

Radial loads produced due to high speed rotation of rotor 304 are muchsmaller than axial loads so that virtually no wear occurs aroundzirconia rotor shaft 302. Even when the pump components weredisassembled for inspection after significant running time, no wearcould be detected during microscopic examination of shaft 302 whereshaft 302 contacts olive hole ring jewel 307. The radial clearancebetween shaft 302 and olive hole ring jewel 307 is preferably in therange of about one ten-thousandths to two ten-thousandths(0.0001-0.0002) of an inch.

Olive hole ring jewel 307 maintains extremely precise impeller rotationand yet allows for axial shaft movement. No pre-load mechanism isrequired because bearing system 300 is designed to run with a smallamount of shaft end-play. The shaft end-play after break-in ispreferably less than about ten thousandths (0.010) of an inch. Fluidpressure is not needed to support bearing loads because the bearingloads at circumference 312 and inner curved surface 330 contact eachother. This bearing system therefore eliminates the need for an externalreservoir or complex fluid porting arrangements or other forced fluidmethods as used with other bearing systems. Blood trauma and potentialof clotting is therefore reduced as compared with bearing systems thatforce blood through small clearances.

Another feature of olive hole ring jewel 307 is a relatively narrowwidth W as indicated in FIG. 13 that permits heat conduction througholive hole ring jewel 307. Width W is measured parallel to axis ofrotation of rotor 304. While commercially available olive hole ringjewels may be purchased with a width of less than thirty-fivethousandths (0.035) of an inch, specially made jewels may be formed withan even smaller width as desired. The advantage of a small width W ofolive ring jewel 307 is that the heat produced by bearing system 300 isnot insulated by jewel 307 so as to continue increasing. Instead, theheat conducts outwardly into the blood to provide a cool runningbearing. The lower bearing temperature reduces the likelihood of bearingseizure, increases bearing life, and reduces the possibility of thrombusformation when blood contacts the bearings. It will be observed that aminimum thickness of jewel 307 is preferably required to support radialforces acting on shaft 302 on circumference 312 and/or to maintain jewel307 in contact with circumference 312 of shaft 302.

Bearing system 300 requires no shaft seals. Instead, the outer surfacesof shaft 302, endstone 305, and olive hole ring jewel 307, define void332 as indicated in FIG. 14A and FIG. 14B. Void 332 may be made verysmall by selecting radius R1 to be significantly larger than radius R2.Any blood that enters void 332 coagulates quickly due to bearing heat toform a smooth and stable surface that conforms to radiused surface 308.Due to the conforming shape, coagulated blood does not interfere withshaft rotation. Once coagulated, the blood cannot leave void 332 toenter the blood stream. Furthermore, any coagulated blood in void 332acts to further stabilize end play of shaft 302. Radius R2 of shaft 302may be made quite small if desired to reduce even further any frictioncreated by coagulated blood. Certain other advantages of a void such asvoid 332 over attempts to seal or wash bearings with saline fluids arediscussed hereinafter in connection with other bearing embodiments.However, FIG. 11 through FIG. 14B disclose the presently preferredbearing system configuration embodiments.

In another bearing embodiment having a void for blood, FIG. 6A disclosesa ball-socket bearing configuration. The bearings are comprised ofbio-compatible material such as ceramic material. The ball-socketbearing 121 may be configured as shown in FIG. 1 or in otherconfigurations. Ball 120 is preferably secured by some securing meanssuch as glue, welding, or other means along edge 128. Ball 120 could bemolded into one component or split and secured as is known in the art.Ball 120 has a spherical surface 122 that engages a mating seatspherical surface 124. A void or bearing chamber 126 is filled withbio-compatible material to prevent blood from coming into this area andstagnating. In this embodiment, bearing chamber 126 is left empty andallowed to fill with blood. The blood cross-links due to bearing heatand takes on a soft, pliable, plastic texture. The cross-linked bloodmay perform, to some extent, a bearing surface function. Thecross-linked blood then prevents other blood from entering the bearingand stagnating.

In an alternative bearing configuration, shaft-journal bearing 130provides that shaft 132 is secured to component 134, which may be rotor46, for rotation with respect to a second component 136, which may bediffuser 20. Journal sleeve 138 has a cylindrical bearing receivingsurface 140 for engaging cylindrical shaft bearing surface 142.Bio-compatible material in bearing chamber 144 is preferablycross-linked blood, as discussed hereinbefore, which has leaked intothis chamber, heated up, and solidified.

Another embodiment of a shaft-journal bearing would include a shaft (notshown) extending through rotor 46 and engaging respective sleeves onflow straightener 18 and diffuser 20. As well, the flow straightenerand/or diffuser, or relevant portions thereof, may be made from asuitable material, such as zirconia, with the bearing surfaces beingformed or machined directly into that material. In a similar manner, thebearing surfaces of bearing 121, 130, or other bearings could bemachined into a bearing mount component in this way.

To avoid the problem of blood stagnation in the region of the bearing,several bearing washing configurations, using directed blood flow orpump differential pressures, may also be used as part of the presentinvention. Referring to FIG. 3, there is shown a means for using a pumppressure differential for washing bearing 150 which includes shaft 152and journal sleeve 153. High pressure region 154 produces a blood flowthrough passage 156 to wash bearing 150 and exit to lower pressureregion 158. Thus, blood is prevented from stagnating behind bearing 150.

Another bearing embodiment (not shown) would include a pressure fedjournal bearing to form a hydrodynamic film that supports the shaft andloads so that the bearing surfaces do not touch. Shaft 152 would beslightly undersize with respect to a journal sleeve for this purpose,and a pressure fed flow passage may be directed through such ahydrodynamic journal bearing from high pressure region 154 to lowpressure region 158.

FIG. 6C illustrates another bearing washing method by producing bend 160in pump housing 12. This places bearing 162 in a high velocity flowarea. Blood flow through bend 160 washes the bearing members whichinclude a male portion 164 and mating member 166. Other configurationsfor this type arrangement may be used but the general principle is asshown. The flow straightener is removed in this embodiment but may beincluded in another embodiment where it does not necessarily need to actas a bearing support.

FIG. 6D and 6E illustrate yet another alternative bearing arrangementembodiment. In this embodiment, a bearing shaft 170 extends throughjournal sleeve 172. Bearing shaft 170 has an oblong, or substantiallyoval transverse cross-section as shown in FIG. 6E. This configurationproduces flow paths 174 through journal sleeve 172 so as to flush thebearing and prevent blood stagnation. Shaft 170 could also be fluted soas to have spirals, slots, or in some other manner form flow paths 174along its periphery within sleeve 172.

FIG. 6F shows a bearing configuration wherein rotor 46 is cantileveredwith respect to diffuser 20 using two bearings 180 and 182. Shaft 184extends between the two bearings. Each bearing includes a rotating ballsurface, 186 and 188, mating with a socket surface 190 and 192,respectively. Socket surfaces 190 and 192 are formed in ceramic materialsleeves 194 and 196, respectively. The bearings are "zero tolerance"bearings. Thus, either shaft 184 which runs through diffuser 20, or thediffuser 20 itself must provide a take up mechanism that keeps each ballsurface 186 and 188 tightly engaged with respective socket surfaces 190and 192. For this purpose, diffuser 20 could be made of compressiblematerial. Alternatively, diffuser 20 could receive injection moldingaround sleeves 194 or 196.

As another alternative, either a passive or active, or a combinationpassive-active magnetic bearing suspension system (not shown) could beused to rotatably mount rotor 46 whereby rotor 46 would be axiallypositioned and/or bearing surfaces would be suspended with respect toeach other using magnetic force.

The method of the present invention is illustrated in FIG. 8 which showsa test matrix for optimizing the pump parameters which are believed toaffect hemolysis. This method enables optimization of pump parameters asdiscussed hereinafter with a minimum number of tests. Although thismatrix is designed for a blood pump with no inducer or flowstraightener, it is believed the method of the present invention isclearly illustrated with this example and may be used to improve mostother blood pump designs with respect to hemolysis, thrombus, pumpefficiency, cost, and other important factors. Using this approach, itis possible to thoroughly investigate many parameters in an organized,methodical approach to achieve highly desired goals such as optimum pumpperformance and minimum hemolysis.

To apply this method, it was first necessary to identify the pumpparameters that were believed to affect hemolysis. The variables arelisted at the headings of the test matrix and include blade tip shape,radial clearance, axial clearance, number of blades, and impellerlength. To judge effectively the impact of each variable on hemolysis,the method of the present invention requires that all pump dimensions beheld constant while only one variable is changed. As shown in the matrixof FIG. 8, 16 different tests are used. Preferably, each test is made atleast three different times to provide statistical results and checkconsistency.

The information learned in initial tests is used later in the matrix.For example, tests 1 and 2 compare the effects of flat and round bladetip geometries. The least hemolytic of the two is then used in allremaining tests. If there is no statistically significant difference,then the result that provides superior hydrodynamic performance is used.If there is no hydrodynamic difference, then the least expensiveparameter to manufacture is used. Test conditions are constant for eachparameter. All tests were made using bovine blood at a flow of 5liters/min against 100 mm Hg. The duration of each test was 20 minutes.The pump circulates blood through a test loop having a 500 cc bloodreservoir which was conveniently a 500 cc blood bag. The 500 cc bloodbag is preferably changed after each version of each parameter istested.

In order to compare various impeller and stator blade geometries withina reasonable time period, a stereolithography technique was used toquickly realize the complex shapes. This technique relies on a laserbeam that scans the surface of a liquid acrylic polymer. The polymerhardens under the influence of the laser, and layer by layer a solidshaped is formed from the liquid surface.

To compare hemolysis results, an index of hemolysis (IH) is used. Thisis defined as the amount of hemoglobin liberated in grams per 100 litersof blood pumped against 100 mm Hg. In equation form: ##EQU1## where:

IH equals amount of hemoglobin liberated in grams per 100 liters ofblood pumped against 100 mm Hg;

Ht is the hematocrit in decimal percent;

V is the blood volume in liters;

ΔHb is the amount of hemoglobin liberated in a fixed time period ingrams/liter;

Flow is the flow rate in liters per minute; and

Time is the total time in minutes at that flow rate.

The final results are seen in the matrix of FIG. 8 and other moredetailed factors of testing in accord with the method of this inventionare discussed more thoroughly in the articles which have been namedpreviously and incorporated herein by reference.

As to manufacturing and usage considerations, Pump 10 is preferablymanufactured using materials designed to be buoyant inside the body tomake the completed pump neutrally buoyant or approximately neutrallybuoyant. This minimizes stress on stitches or other means used toposition the pump within the body. Thus, the rotor, rotor blades, and/orother components may be made with lightweight material having sufficientthickness to produce a buoyant effect.

Pump 10 has numerous uses as a blood pump including use as a portableblood pumping unit for field service or for other clinical applicationsinvolving other fluids. It could, for instance, be used in a compactheart-lung machine. Due to the small volume and size of pump 10, it canbe placed close to a patient to minimize shock caused when initiatingblood pump operation using saline solution. Larger pumps, with largervolume, may be awkward to move close by a patient to eliminate thisshock.

Thus, the blood pump of the present invention, optimized using themethod of the present invention has many advantages over the prior art.For instance, there are no blood seals which require bearing purgesystems. The bearing system has been found to require no maintenance forlong-term, reliable operation. No Hall Effect sensors are required whichmay tend to limit motor control reliability due to their complexity. Aswell, pump 10 provides low power consumption and produces very lowlevels of hemolysis.

The foregoing disclosure and description of the invention isillustrative and explanatory thereof, and it will be appreciated bythose skilled in the art, that various changes in the size, shape andmaterials as well as in the details of the illustrated construction,reliability configurations, or combinations of features of the variousrotary pump elements of the present invention may be made withoutdeparting from the spirit of the invention.

What is claimed is:
 1. A fluid pump, comprising:a pump housing definingtherein a fluid path for fluid flow through said pump; a rotor mountedwithin said pump housing for rotation therein; at least one impellerblade on said rotor; at least one flat ceramic endstone for supportingaxial loads acting on said rotor; a plurality of magnets secured to saidrotor; and a stator in surrounding relationship to said pump housing,said stator and said plurality of magnets being positioned to define agap therebetween.
 2. The pump of claim 1, further comprising:a shaftwith a shaft end for supporting said rotor:a jewel bearing having anaperture therethrough with a curved inner surface for receiving saidshaft end and supporting said shaft end with near line contact around acircumference of said shaft end.
 3. The pump of claim 1, wherein:saidplurality of magnets are arranged in helical fashion about said rotor.4. The pump of claim 1, wherein:said stator is axially adjustable withrespect to said pump housing and said rotor.
 5. The pump of claim 1,further comprising:a radially peripheral edge of said at least oneimpeller blade, said plurality of magnets being mounted along saidradially peripheral edge.
 6. A rotary motor, comprising:a motor housing;a rotor mounted within said motor housing for rotation about a rotorshaft, said rotor having a circumference; a stator disposed mounted tosaid pump housing to produce a magnetic field for rotatably driving saidrotor; a bearing with a substantially torroidial inner surface forsupporting said rotor shaft; and a plurality of individual permanentmagnets positioned along said circumference of said rotor with avariable axial position.
 7. The rotary motor of claim 6, furthercomprising:a radiused shaft end for said rotor shaft; and an endstonewith a flat planar surface for engaging said radiused shaft end.
 8. Therotary motor of claim 6, wherein:said plurality of individual permanentmagnets are mounted with a radial spacing to said stator of less than0.025 inches.
 9. The rotary motor of claim 6, further comprising:atleast one impeller blade mounted to said rotor, said plurality ofindividual magnets being affixed along a peripheral edge of said atleast one impeller blade.
 10. The rotary motor of claim 6, wherein saidstator further comprises:a plurality of laminations, said laminationsbeing stacked together such that a passageway is formed therein, saidpassageway being skewed with respect to said rotor shaft.
 11. The rotarymotor of claim 6, wherein:said at least one impeller blade has avariable pitch.
 12. A rotary motor, comprising:a motor housing; a rotormounted within said motor housing for rotation about a rotor shaft, saidrotor having a circumference; a stator mounted to said pump housing toproduce a magnetic field for rotatably driving said rotor; ceramiccontact bearings positioned axially and radially, with respect to saidrotor shaft, to support axial and radial loads; a plurality ofindividual permanent magnets positioned along said circumference of saidrotor; and a stator mounted to said motor housing comprised of aplurality of laminations.
 13. The rotary motor of claim 12, furthercomprising:a shaft end formed of a first material, said axiallypositioned ceramic bearing being formed of a second ceramic materialharder than said first material, such that substantially all wear occurson said shaft end.
 14. The rotary motor of claim 12, wherein:said statoris axially adjustable with respect to said pump housing.
 15. The rotarymotor of claim 12, further comprising:a shaft end having a radiused end,said axially positioned ceramic bearing being an endstone with a flatplanar surface for engaging said shaft end, said radially positionedbearing being an olive hole ring jewel for receiving said shaft end. 16.The rotary motor of claim 12, wherein:said plurality of individualpermanent magnets are mounted at different axial positions with respectto said rotor shaft such that said plurality of individual permanentmagnets define at least a portion of a helical pattern.